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RFC 5690

Adding Acknowledgement Congestion Control to TCP

Pages: 33
Informational

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Independent Submission                                          S. Floyd
Request for Comments: 5690                                          ICIR
Category: Informational                                         A. Arcia
ISSN: 2070-1721                                                   D. Ros
                                                        TELECOM Bretagne
                                                              J. Iyengar
                                             Franklin & Marshall College
                                                           February 2010


            Adding Acknowledgement Congestion Control to TCP

Abstract

This document describes a possible congestion control mechanism for acknowledgement (ACKs) traffic in TCP. The document specifies an end-to-end acknowledgement congestion control mechanism for TCP that uses participation from both TCP hosts: the TCP data sender and the TCP data receiver. The TCP data sender detects lost or Explicit Congestion Notification (ECN)-marked ACK packets, and tells the TCP data receiver the ACK Ratio R to use to respond to the congestion on the reverse path from the data receiver to the data sender. The TCP data receiver sends roughly one ACK packet for every R data packets received. This mechanism is based on the acknowledgement congestion control in the Datagram Congestion Control Protocol's (DCCP's) Congestion Control Identifier (CCID) 2. This acknowledgement congestion control mechanism is being specified for further evaluation by the network community. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5690.
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IESG Note

   The content of this RFC was at one time considered by the IETF, and
   therefore it may resemble a current IETF work in progress or a
   published IETF work.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

1. Introduction ....................................................3 2. Conventions and Terminology .....................................4 3. Overview ........................................................4 4. Acknowledgement Congestion Control ..............................6 4.1. The ACK Congestion Control Permitted Option ................6 4.2. The TCP ACK Ratio Option ...................................7 4.3. The Receiver: Implementing the ACK Ratio ...................7 4.4. The Sender: Determining Lost or Marked ACK Packets .........8 4.4.1. The Sender: Detecting Lost ACK Packets after a Congestion Event ...........................10 4.5. The Sender: Adjusting the ACK Ratio .......................10 4.5.1. Possible Addition: Decreasing the ACK Ratio after a Congestion Window Decrease .................12 4.6. The Receiver: Sending ACKs for Out-of-Order Data Segments ..................................................12 4.7. The Sender: Response to ACK Packets .......................13 4.8. Possible Addition: Receiver Bounds on the ACK Ratio .......15 5. Possible Complications .........................................15 5.1. Possible Complication: Delayed Acknowledgements ...........15 5.2. Possible Complication: Duplicate Acknowledgements .........15 5.3. Possible Complication: Two-Way Traffic ....................16 5.4. Possible Complication: Reordering of ACK Packets ..........16 5.5. Possible Complication: Abrupt Changes in the ACK Path .....17 5.6. Possible Complication: Corruption .........................17 5.7. Possible Complication: ACKs That Don't Contribute to Congestion .............................................17 5.8. Possible Complication: TCP Implementations that Skip ACK Packets ..........................................20
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      5.9. Possible Complication: Router or Middlebox-Based
           ACK Mechanisms ............................................21
      5.10. Possible Complication: Data-Limited Senders ..............21
      5.11. Other Issues .............................................22
   6. Evaluating ACK Congestion Control ..............................22
      6.1. Contention in Wireless Links or in Non-Switched Ethernet ..22
      6.2. Keep-Alive and Other Special ACK Packets ..................22
   7. Measurements of ACK Traffic and Congestion .....................23
   8. Acknowledgement Congestion Control in DCCP's CCID 2 ............23
   9. Security Considerations ........................................24
   10. IANA Considerations ...........................................25
   11. Conclusions ...................................................26
   12. Acknowledgements ..............................................26
   Appendix A. Related Work ..........................................27
      A.1. ECN-Only Mechanisms .......................................28
      A.2. Receiver-Only Mechanisms ..................................28
      A.3. Middlebox-Based Mechanisms ................................29
   Appendix B. Design Considerations .................................29
      B.1. The TCP ACK Ratio Option, or an AckNow Bit in
           Data Packets? .............................................29
   Normative References ..............................................30
   Informative References ............................................30

1. Introduction

This document describes a congestion control mechanism for acknowledgements (ACKs) to TCP. This mechanism is based on the acknowledgement congestion control in DCCP's CCID 2 ([RFC4340], [RFC4341]), which is a successor to the TCP acknowledgement congestion control mechanism proposed by Balakrishnan, et al. in [BPK97]. In this document we use the terminology of senders and receivers, with the sender sending data traffic and the receiver sending acknowledgement traffic in response. In CCID 2's acknowledgement congestion control, specified in Section 6.1 of [RFC4341], the receiver uses an ACK Ratio R reported to it by the sender, sending roughly one ACK packet for every R data packets received. The CCID 2 sender keeps the acknowledgement rate roughly TCP-friendly by monitoring the acknowledgement stream for lost and marked ACK packets and modifying the ACK Ratio accordingly. For every round-trip time (RTT) containing an ACK congestion event (that is, a lost or marked ACK packet), the sender halves the acknowledgement rate by doubling the ACK Ratio; for every RTT containing no ACK congestion event, the sender additively increases the acknowledgement rate through gradual decreases in the ACK Ratio.
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   The goal of this document is to explore a similar congestion control
   mechanism for acknowledgement traffic for TCP.  The assumption is
   that in some environments with congestion on the reverse path,
   reducing the sending rate for ACK traffic traversing the congested
   path can help to reduce the congestion itself.  For those
   environments where the reverse path is congested but where TCP ACK
   traffic does not appreciably contribute to that aggregate congestion,
   the goal is for TCP's ACK congestion control to have a minimal
   negative effect on the performance of the TCP connection.

   Adding acknowledgement congestion control as an option in TCP would
   require the following:

   * An agreement from the TCP hosts on the use of ACK congestion
     control.  For the mechanism specified in this document, the TCP
     hosts would use a new TCP option, the ACK Congestion Control
     Permitted option.

   * A mechanism for the TCP sender to detect lost and ECN-marked pure
     acknowledgement packets.

   * A mechanism for adjusting the ACK Ratio.  The TCP sender would
     adjust the ACK Ratio as specified in Section 6.1.2 of [RFC4341].

   * A method for the TCP sender to inform the TCP receiver of a new
     value for the ACK Ratio.  For the mechanism specified in this
     document, the TCP sender would use a new TCP option, the ACK Ratio
     option.

2. Conventions and Terminology

MSS refers to the Maximum Segment Size. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Overview

This section gives an overview of acknowledgement congestion control for TCP.
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        ---------------------------------------------------------------
        TCP Host A                Router                     TCP Host B
        (data sender)                                   (data receiver)
        ----------                ------                     ----------
                                         <--- SYN with AckCC Permitted.
        SYN/ACK with AckCC Permitted --->
                                  . . .
        Data packets --->
                                                    <--- one ACK packet
                                             for every two data packets
                                  . . .
        Sender detects a lost ACK packet.
        Data packet with an ACK Ratio option of 4 --->
                                                    <--- one ACK packet
                                    for at most every four data packets
                                  . . .
        Sender detects a period with no lost ACK packets.
        Data packet with an ACK Ratio option of 3 --->
                                                    <--- one ACK packet
                                   for at most every three data packets
        ---------------------------------------------------------------

               Figure 1: Acknowledgement Congestion Control,
     Host B as the Connection Initiator, for a Connection without ECN

   Figure 1 gives an example of acknowledgement congestion control
   (AckCC) with TCP Host B as the connection initiator.

   During connection initiation, TCP host B sends an ACK Congestion
   Control Permitted option on its SYN or SYN/ACK packet.  This allows
   TCP host A (now called the sender) to send instructions to TCP host B
   (now called the receiver) about the ACK Ratio to use in responding to
   data packets.

   Also during connection initiation, TCP host A sends an ACK Congestion
   Control Permitted option on its SYN or SYN/ACK packet.  In
   combination with TCP host B's sending of an ACK Congestion Control
   Permitted option, and with the negotiation of ECN-Capability as
   specified in [RFC3168], this would allow TCP host B to send its ACK
   packets as ECN-Capable.

   The TCP receiver starts with an ACK Ratio of two, generally sending
   one ACK packet for every two data packets received.
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   The TCP sender detects a lost or ECN-marked ACK packet from the TCP
   receiver and sends an ACK Ratio option of four to the receiver.  The
   TCP receiver changes to an ACK Ratio of four, sending one ACK packet
   for at most four data packets.  The TCP sender uses Appropriate Byte
   Counting and rate-based pacing in responding to these ACK packets.

   The TCP sender detects a period with no lost ACK packets and sends an
   ACK Ratio option of three to the TCP receiver.  The TCP receiver
   changes back to an ACK Ratio of three, sending one ACK packet for at
   most three data packets.

4. Acknowledgement Congestion Control

The goal of the mechanism proposed in this document is to control pure ACK traffic on the path from the TCP data receiver to the TCP data sender. Note that the approach outlined here is an end-to-end one (as is the approach followed by DCCP's CCID 2 [RFC4341]), but it may also take advantage of explicit congestion information from the network, conveyed by ECN [RFC3168], if available. The ECN specification ([RFC3168], see Section 6.1.4) prohibits a TCP receiver from setting the ECT(0) or ECT(1) codepoints in IP packets carrying pure ACKs, but *only* as long as the receiver does *not* implement any form of ACK congestion control. Unlike some of the related work cited in the appendix, in this document we are proposing an end-to- end ACK congestion control mechanism that controls congestion on the reverse path (the path followed by the ACK traffic) by detecting and responding to either marked or dropped ACK packets.

4.1. The ACK Congestion Control Permitted Option

The TCP end-points would negotiate the use of ACK congestion control (AckCC) with a TCP option: the ACK Congestion Control Permitted option. The option number would be allocated by IANA. The ACK Congestion Control Permitted option can only be sent on packets that have the SYN bit set. If TCP end-point A receives an ACK Congestion Control Permitted option from TCP end-point B, then the TCP end-points may use ACK congestion control on the pure acknowledgements sent from B to A. This means that TCP end-point A may send ACK Ratio values to TCP end-point B, for TCP end-point B to use on pure acknowledgement packets. Equivalently, if TCP end-point A *does not* receive an ACK Congestion Control Permitted option from TCP end-point B, then TCP end-point A knows not to waste its time detecting lost ACK packets and adjusting and sending the ACK Ratio values.
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   If TCP end-point B receives an ACK Congestion Control Permitted
   option from TCP end-point A, then the TCP end-points may use ACK
   congestion control on the pure acknowledgements sent from A to B.

   If TCP end-point B receives an ACK Congestion Control Permitted
   option from TCP end-point A and also sent an ACK Congestion Control
   Permitted option to TCP end-point A, and if ECN-Capability has been
   negotiated, then TCP end-point B can send its pure ACK packets as
   ECN-Capable.

          TCP ACK Congestion Control Permitted Option:

          Kind: TBD1

          +-----------+-----------+
          | Kind=TBD1 |  Length=2 |
          +-----------+-----------+

   When ACK congestion control is used, the default initial ACK Ratio is
   two, with the receiver acknowledging at least every other data
   packet.

4.2. The TCP ACK Ratio Option

The sender uses an ACK Ratio TCP option to communicate the ACK Ratio value from the sender to the receiver. TCP ACK Ratio Option: Kind: TBD2 +-----------+-----------+-----------+ | Kind=TBD2 | Length=3 | ACK Ratio | +-----------+-----------+-----------+ The ACK Ratio option is only sent on data packets. Because TCP uses reliable delivery for data packets, the TCP sender can tell if the TCP receiver has received an ACK Ratio option.

4.3. The Receiver: Implementing the ACK Ratio

With an ACK Ratio of R, the receiver should send one pure ACK for every R newly received data packets unless the delayed ACK timer expires first. A receiver could simply maintain a counter that increments by one for each new data packet received, and send an ACK packet when the counter reaches R. The receiver would reset the counter to zero whenever a pure or piggybacked ACK is sent.
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   If the receiver has buffer limitations, the receiver might have to
   acknowledge K packets, for some K less than the current ACK Ratio R.
   In this case, the sender could observe from the acknowledgements that
   the receiver is acknowledging less than R packets.

   It is possible for there to be lost or marked ACK packets when there
   haven't yet been any lost or marked data packets.  Thus, the sender
   could increase the ACK Ratio R even during the initial slow-start.

   [RFC5681] recommends that the receiver SHOULD acknowledge out-of-
   order data packets immediately, sending an immediate duplicate ACK
   when it receives a data segment above a gap in the sequence space,
   and sending an immediate ACK when it receives a data segment that
   fills in all or part of a gap in the sequence space.

   When ACK congestion control is being used and the ACK Ratio is at
   most two, the TCP receiver acknowledges each out-of-order data packet
   immediately.  For an ACK Ratio greater than two, Section 4.6
   specifies in detail the receiver's behavior for sending ACKs for out-
   of-order data packets.

4.4. The Sender: Determining Lost or Marked ACK Packets

The TCP data sender uses its knowledge of the ACK Ratio in use by the receiver to infer when an ACK packet has been lost. Because the TCP sender knows the ACK Ratio R in use by the receiver, the TCP sender knows that in the absence of dropped or reordered acknowledgement packets, each new acknowledgement received will acknowledge at most R additional data packets. Thus, if the sender receives an acknowledgement acknowledging more than R data packets, and does not receive a subsequent acknowledgement acknowledging a strict subset (with a smaller cumulative acknowledgement, or with the same cumulative acknowledgement but a strict subset of data acknowledged in selective acknowledgement (SACK) blocks), then the sender can infer that an ACK packet has been dropped. The use of SACK options in ACK packets would help the sender in detecting lost ACK packets. Similarly, the TCP sender knows that in the absence of dropped or delayed data packets from the sender, and in the absence of delayed acknowledgements due to a timer expiring at the receiver, each new pure acknowledgement received will acknowledge at least R additional data packets. In terms of ACK congestion control, the TCP sender does not have to take any actions when it receives an acknowledgement acknowledging less than R additional packets.
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   Out-of-order data packets:

      If the ACK Ratio is at most two, then the TCP receiver sends a
      duplicate acknowledgement (DupACK) for every out-of-order data
      packet.  In this case, the TCP sender should be able to detect
      lost DupACK packets by counting the number of DupACKs that arrive
      between the beginning of the loss event and the arrival of the
      first full or partial ACK, and comparing this number with the
      number of DupACKs that should have arrived (based on the number of
      packets being ACKed by the full or partial ACK).  Simulations
      and/or experiments will be needed to determine whether, in
      practice, it works for the TCP sender to assess lost ACK packets
      during loss events, for an ACK Ratio of at most two.

      If the ACK Ratio is greater than two, the TCP receiver does not
      send a DupACK for every out-of-order data packet, as specified in
      Section 4.6.  For simplicity, the TCP sender does not attempt to
      detect lost ACK packets during loss events involving forward-path
      data traffic.  That is, as soon as the sender infers a packet loss
      for a forward-path data packet, it stops detection of ACK loss on
      the reverse path.  The sender waits until a new cumulative
      acknowledgement is received that covers the retransmitted data,
      and then restarts detection of ACK loss for reverse-path traffic.

   Detecting lost ACK packets after changes in the ACK Ratio:

      In detecting lost ACK packets, the sender relies on its knowledge
      of the ACK Ratio used by the receiver.  But when the sender makes
      a change in the ACK Ratio and then receives ACK packets, how does
      the sender know whether the receiver was using the new or the old
      ACK Ratio when it sent those ACK packets?  As specified in the
      next section, the sender can make only one of two possible changes
      to the ACK Ratio within one round-trip time.  The sender can
      decrease the ACK Ratio by one, from R to R-1, or the sender can
      double the ACK Ratio, increasing it from R to 2R.  But, in
      detecting lost ACK packets after an increase in the ACK Ratio, the
      sender needs to know whether the receiver was using the old ACK
      Ratio R or the new ACK Ratio 2R.

      The sender sends ACK Ratio options only on data packets, and these
      data packets are acknowledged by the receiver.  One possibility
      would be for the sender to save the sequence number of the last
      data packet that contained an ACK Ratio option and to remember
      whether that ACK Ratio option was for an increase or a decrease in
      the ACK Ratio.  Then, if the sender receives an ACK packet
      acknowledging the saved sequence number, the sender knows that the
      receiver has begun using the new ACK Ratio.
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      It *might* be sufficient for the sender just to save the
      information of whether the last change in the ACK Ratio was an
      increase or a decrease, without saving the sequence number
      associated with the last ACK Ratio option.  In this way, if the
      sender recently increased the ACK Ratio from R to 2R, the sender
      could be more cautious in detecting lost ACK packets.  Another
      possibility would be that, after sending an ACK Ratio option, the
      sender waits until that data has been ACKed, with the new ACK
      Ratio in use by the receiver, before resuming the detection of
      lost ACK packets.  However, we do not explore either of these
      approaches in more detail in this document.

4.4.1. The Sender: Detecting Lost ACK Packets after a Congestion Event

After a sender's retransmit timeout or fast retransmit, the sender might retransmit a number of data packets dropped from a single window of data. In particular, during a loss recovery period (from the sender's detection of the congestion event up until the sender receives an acknowledgement of all data packets transmitted before the loss recovery period began), retransmitted data packets can fill holes in the receiver's sequence space, resulting in irregular jumps in the cumulative acknowledgement field in ACK packets from the receiver. These jumps in the cumulative acknowledgement field make it difficult for the sender to reliably detect lost ACK packets during a loss recovery period. Because of this uneven progress of the cumulative acknowledgement field during a loss recovery period, the sender should not attempt to detect lost ACK packets during a loss recovery period. As a consequence, the sender will not increase the ACK Ratio in response to ACK packets that are lost during a loss recovery period.

4.5. The Sender: Adjusting the ACK Ratio

The TCP sender will adjust the ACK Ratio as specified in Section 6.1.2 of [RFC4341], as follows. The ACK Ratio always meets the following three constraints. (1) The ACK Ratio is an integer. (2) The minimum ACK sending rate: The ACK Ratio does not exceed max(2, cwnd/(K*MSS)), rounded up, for K=2. As a result, the TCP receiver generally sends at least two ACKs in response to a window of at least four full-sized segments.
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   (3) If the congestion window is at least as large as four full-sized
       segments, then the ACK Ratio is at least two.  In other words, an
       ACK Ratio of one is only allowed when the congestion window is at
       most three full-sized segments.

   The sender changes the ACK Ratio within those constraints as follows.

   For each congestion window of data with lost or marked ACK packets,
   the ACK Ratio R is doubled; for each cwnd/(MSS*(R^2 - R)) consecutive
   congestion windows of data with no lost or marked ACK packets, the
   ACK Ratio is decreased by 1.  (See Appendix A of RFC 4341 for the
   derivation.  Note that Appendix A of RFC 4341 assumes a congestion
   window W in packets, while we use cwnd in bytes.)  As stated in the
   previous section, when the ACK Ratio is greater than two, the sender
   does not attempt to detect lost ACK packets during loss events for
   forward-path traffic.

   For a constant congestion window, these modifications to the ACK
   Ratio give an ACK sending rate that is roughly TCP-friendly.  Of
   course, cwnd usually varies over time; the dynamics will be rather
   complex, but roughly TCP friendly.  We recommend that the sender
   determines when to decrease the ACK Ratio by one (i.e., by
   calculating the number of in-order data packets to count) right after
   an ACK loss event.

   The frequency of ACK Ratio negotiations:

      The sender need not keep the ACK Ratio completely up to date.  For
      instance, it may rate-limit ACK Ratio renegotiations to once every
      four or five round-trip times, or to once every second or two.
      The sender should not attempt to change the ACK Ratio more than
      once per round-trip time.  In particular, before sending a packet
      with a new value for the ACK Ratio, the sender should verify that
      the receiver has acknowledged a data packet containing an ACK
      Ratio option for the old value of the ACK Ratio.  Additionally,
      the sender may enforce a minimum ACK Ratio of two, or it may set
      the ACK Ratio to one for half-connections with persistent
      congestion windows of 1 or 2 packets.

   The minimum ACK sending rate:

      From rule (2) above, the TCP receiver always sends at least K=2
      ACKs for a window of data, even in the face of very heavy
      congestion on the reverse path.  We would note, however, that if
      congestion is sufficiently heavy, all the ACK packets are dropped,
      and then the sender falls back on an exponentially backed-off
      timeout.  Thus, if congestion is sufficiently heavy on the reverse
      path, then the sender reduces its sending rate on the forward
Top   ToC   RFC5690 - Page 12
      path, which reduces the rate on the reverse path as well.  One
      possibility would be to use a higher minimum ACK-sending rate,
      adding a constant upper bound on the ACK Ratio.  That is, if the
      ACK Ratio also had an upper bound of J, independent of cwnd, then
      the receiver would always send at least one ACK for every J data
      packets, regardless of the level of congestion on the reverse
      path.

4.5.1. Possible Addition: Decreasing the ACK Ratio after a Congestion Window Decrease

After a lost or ECN-marked data packet, the data sender halves the congestion window, thus halving the sending rate for data packets, while making no change to the ACK Ratio R. As a result, after a congestion event involving a data packet, the sending rate for ACK packets on the return path is also halved. If the congestion event was a lost or ECN-marked data packet, this was due to congestion on the forward path, which may have been unrelated to conditions on the reverse path. Thus, it has been suggested that the sender could decrease the ACK Ratio R when it halves the congestion window; in this case, the halving of the sending rate for data packets would not be accompanied by a halving of the sending rate for ACK packets also. However, there are a few cases where a congestion event involving data packets could in fact have been caused by congestion on the reverse path. As one example, the path could include a congested multiaccess link where forward-path and reverse-path traffic can interfere with each other. Thus, in this case it might be desirable if a congestion event resulted in a reduction in the sending rate of ACK packets as well as of data packets. As a second example of a congestion event involving congestion of the reverse path, a congestion event could be caused not by a dropped or ECN-marked data packet, but by a window of dropped ACK packets, resulting in a retransmit timeout at the data sender. After a retransmit timeout, the TCP sender will slow-start, reducing the congestion window to the initial window and setting the ACK Ratio to at most two. Until further investigation, the sender will not decrease the ACK Ratio as a result of a congestion event involving a data packet.

4.6. The Receiver: Sending ACKs for Out-of-Order Data Segments

RFC 5681 says that "a TCP receiver SHOULD send an immediate duplicate ACK when an out-of-order segment arrives". After three duplicate ACKs are received, the TCP sender infers a packet loss and implements
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   fast retransmit and fast recovery, retransmitting the missing packet.
   When the ACK Ratio is at most two, the TCP receiver should still send
   an immediate duplicate ACK when an out-of-order segment arrives.

   In general, when the ACK Ratio is greater than two, the TCP receiver
   still should send an immediate duplicate ACK for each of the first
   three out-of-order segments that arrive in a reordering event.  (We
   define a reordering event at the receiver as beginning when an out-
   of-order segment arrives, and ending when the receiver holds no more
   out-of-order segments.)  However, when the ACK Ratio is greater than
   two, after the first three duplicate ACKs have been sent, the TCP
   receiver should perform ACK congestion control on the remaining ACKs
   to be sent during the current reordering event.  That is, after the
   first three duplicate ACKs have been sent, the TCP receiver should
   return to sending an ACK for every R segments, instead of sending an
   ACK for every out-of-order segment in that reordering event.  (We
   note that the fast recovery procedure of the TCP sender might have to
   be modified to take this change into account.)  In addition, a
   receiver must not withhold an ACK for more than 500 ms.

   We note that in an environment with systematic reordering in the data
   path (e.g., every set of K data packets arrives in inverted order,
   for some value of K), the guideline above could result in the
   receiver sending an ACK for every data packet, regardless of the ACK
   Ratio.  In such an environment with persistent reordering, the
   receiver may decide not to send an immediate duplicate ACK for each
   of the first three out-of-order segments that arrive in a reordering
   event.  We leave the investigation of mechanisms for effective ACK
   congestion control in environments with systematic reordering for
   future work.

4.7. The Sender: Response to ACK Packets

The use of a large ACK Ratio can generate line-rate data bursts at a TCP sender. When the ACK Ratio is greater than two, the TCP sender should use some form of burst mitigation or rate-based pacing for sending data packets in response to a single acknowledgement. The use of rate-based pacing will be limited by the timer granularity at the TCP sender. We note that the interaction of ACK congestion control and burst mitigation schemes needs further study. Byte counting at the sender: In addition to the impact of a large ACK Ratio on the burstiness of the TCP sender's sending rate, a large ACK Ratio can also affect the data-sending rate by slowing down the increase of the
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      congestion window cwnd.  As specified in RFC 5681, in slow-start
      the TCP sender increases cwnd by one full-sized segment for each
      new ACK received (in this context, a "new ACK" is an ACK that
      acknowledges new data).  RFC 5681 also specifies that in
      congestion avoidance, the TCP sender increases cwnd by roughly
      1/cwnd full-sized segments for each ACK received, resulting in an
      increase in cwnd of roughly one full-sized segment per round-trip
      time.  In this case, the use of a large ACK Ratio would slow down
      the increase of the sender's congestion window.

      RFC 5681 notes that during congestion avoidance, it is also
      acceptable to count the number of bytes acknowledged by new ACKs
      and to increase cwnd based on the number of bytes acknowledged,
      rather than on the number of new ACKs received.  Thus, the sender
      should use this form of byte counting with acknowledgement
      congestion control, so that the acknowledgement congestion control
      doesn't slow down the window increases for the data traffic sent
      by the sender.  Because rate-based pacing should be used with
      acknowledgement congestion control, as recommended earlier in this
      section, the TCP sender may increase the congestion window by more
      than two MSS for each ACK.

      We note that for Appropriate Byte Counting (ABC) as specified in
      [RFC3465], during slow-start the sender is allowed to increase the
      congestion window by at most two MSS for each ACK.  It has not yet
      been determined whether, with acknowledgement congestion control,
      the TCP sender could use ABC during slow-start.  If ABC is used
      with acknowledgement congestion control, then when the TCP sender
      is in slow-start and the ACK Ratio is greater than two, the TCP
      sender may increase the congestion window by more that two MSS in
      response to a single ACK.  Section 4.2 of [LL07] explores some of
      the issues with the use of ABC for TCP connections with a fixed
      ACK Ratio greater than two.

   Inferring lost data packets:

      As cited earlier, RFC 5681 infers that a packet has been lost
      after it receives three duplicate acknowledgements.  Because ACK
      congestion control is only used when there is congestion on the
      reverse path, after a packet loss, one or more of the three
      duplicate ACKs sent by the receiver could be lost on the reverse
      path, and the receiver might wait until it has received R more
      out-of-order segments before sending the next duplicate ACK.  All
      this could slow down fast recovery and fast retransmit quite a
      bit.  The use of SACK can help reduce the potential delay in
      detecting a lost packet.  With SACK, a TCP sender can use the
      information in the SACK option to detect when the receiver has
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      received at least three out-of-order data packets and to initiate
      fast retransmit and fast recovery in this case, even if the TCP
      sender has not yet received three duplicate ACKs.

4.8. Possible Addition: Receiver Bounds on the ACK Ratio

It has been suggested that in some environments, the TCP receiver might want to set lower bounds on the ACK Ratio. For example, the TCP receiver might know from configuration or from past experience that the bandwidth on the return path is limited, and might want to set a lower bound (greater than two) on the ACK Ratio R. If this is included, this would require a TCP option from the TCP receiver to the TCP sender, reporting the lower bound on the ACK Ratio. Care would also be needed so that the lower bound on the ACK Ratio was only in effect when the TCP sender's congestion window was sufficiently high.

5. Possible Complications

5.1. Possible Complication: Delayed Acknowledgements

The receiver could send a delayed acknowledgement acknowledging a single packet, even when the ACK Ratio is two or more. This should not cause false positives (when the TCP sender infers a loss when no loss happened). The TCP sender only infers that a pure ACK packet has been lost when no data packet has been lost and an ACK packet arrives acknowledging more than R new packets. Delayed acknowledgements could, however, cause false negatives, with the TCP sender unable to detect the loss of an ACK packet sent as a delayed acknowledgement. False negatives seem acceptable; this would result in approximate ACK congestion control, which would be better than no ACK congestion control at all. In particular, when this form of false negative occurs, it is because the receiver is sending acknowledgements at such a low rate that it is sending delayed acknowledgements, rather than acknowledging at least R data packets with each acknowledgement.

5.2. Possible Complication: Duplicate Acknowledgements

As discussed in Section 4.3, RFC 5681 states that "a TCP receiver SHOULD send an immediate duplicate ACK when an out-of-order segment arrives", and that "a TCP receiver SHOULD send an immediate ACK when the incoming segment fills in all or part of a gap in the sequence space" [RFC5681]. When ACK congestion control is used, the TCP receiver instead uses the guidelines from Section 4.6 to govern the sending of duplicate ACKs. More work would be useful to evaluate the
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   advantages and disadvantages of this approach in terms of the
   potential delay in triggering fast retransmit, and to explore
   alternate possibilities.

5.3. Possible Complication: Two-Way Traffic

In a TCP connection with two-way traffic, the receiver could send some pure ACK packets and some acknowledgements piggybacked on data packets. The receiver would still follow the rule of only sending a pure ACK packet when there is a need for a delayed ACK or when there are R new data packets to acknowledge. In a connection with two-way traffic, the TCP sender would not always be able to infer when a pure ACK packet had been lost. For example, the receiver could send a pure ACK packet acknowledging packet K and, soon afterwards, the receiver could send a newly generated data packet for the reverse-path flow also acknowledging packet K. The pure ACK packet could be dropped in the network, and the sender would not be able to detect this drop. Fortunately, there are limitations to the potential problems caused by undetected ACK losses in two-way traffic. The sender will only fail to detect the loss of a pure ACK packet if the ACK packet was followed by a data packet with the same acknowledgement number. If the reverse-path traffic for the connection is dominated by data traffic, then the congestion control for the data traffic is more important than the congestion control for the pure ACK traffic. If the reverse-path traffic is dominated by pure ACK traffic, then the sender would detect any losses of pure ACK packets followed by other pure ACK packets, and this would include most of the pure ACK packets for that connection. Thus, the sender's failure to detect the loss of a pure ACK packet followed by a data packet with the same acknowledgement number would not disable acknowledgement congestion control for a TCP connection with two-way traffic.

5.4. Possible Complication: Reordering of ACK Packets

It is possible for ACK packets to be reordered on the reverse path. The TCP sender could either use a parallel mechanism to the DupACK threshold to infer when an ACK packet has been lost, as with TCP, or, more robustly, the TCP sender could wait an entire round-trip time before inferring that an ACK packet has been lost [RFC4653].
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5.5. Possible Complication: Abrupt Changes in the ACK Path

What happens when there are abrupt changes in the reverse path, such as from vertical handovers? Can there be any problems that would be worse than those experienced by a TCP connection that is not using ACK congestion control?

5.6. Possible Complication: Corruption

As with data packets, it is possible for ACK packets to be dropped in the network due to corruption rather than congestion. The current assumption of ACK congestion control is that all losses should be taken as indications of congestion. If there is ever some better mechanism for identifying and responding to corrupted TCP data packets, the same solution hopefully would apply to corrupted ACK packets as well. One problem with the interaction of packet corruption and congestion control, for both data and ACK packets, is that it is not always obvious when the packet corruption is related to congestion and when the packet corruption is independent of the level of congestion on the corrupting link. In environments where packet corruption exists and is independent of the level of congestion on the corrupting link, applying ACK congestion control would only make the connection more sensitive to ACK packet corruption by reducing the number of ACKs that are sent.

5.7. Possible Complication: ACKs that Don't Contribute to Congestion

It is possible for the ACK packets in a TCP connection to traverse a congested path where ACK packets are dropped but where the ACK packets themselves don't significantly contribute to the congestion on the path. In scenarios where ACK packets are dropped but where ACK traffic doesn't make a significant contribution of the congestion on the path, the use of ACK congestion control would not contribute to reducing the aggregate congestion on the path. In this case, one goal is to minimize the negative impact of ACK congestion control on the overall performance of the TCP connection. J TCP conns. link L -> J TCP conns. data -> |---| |---| <- ACKs <-------------> | | | | <-------------> | | <-------------> | | <-------------> | | | | <-------------> K TCP conns. |---| |---| K TCP conns. ACKs -> <- link L1 <- data Figure 2. A Scenario with J Forward and K Reverse TCP Connections
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   To explore the relative contribution of ACK traffic on congestion, it
   is useful to consider a simple scenario with a congested
   unidirectional link L carrying data traffic from J TCP connections
   (the forward TCP connections) and ACK traffic from K TCP connections
   (the reverse TCP connections).  We assume that all TCP connections
   have the same round-trip time R and the same data packet size S of
   1500 bytes.  We further assume that all of the forward TCP
   connections have the same data packet drop rate p and the same
   congestion window W, and that all of the reverse TCP connections have
   the same congestion window W1 and the same ACK packet drop rate p1.
   (The packet drop rate for data packets is defined as the fraction of
   arriving data packets that are dropped; similarly, the packet drop
   rate for ACK packets is the fraction of arriving ACK packets that are
   dropped.)  The J TCP connections each use a bandwidth on link L of
   1500*W/R bytes per second, and the K TCP connections, without ACK
   congestion control, each use a bandwidth on link L of 40*(W1/2)/R
   bytes per second.  This gives a ratio of 75*(J/K)*(W/W1) for TCP data
   bandwidth to TCP ACK bandwidth on link L.  The ratio J/K is the ratio
   between the number of forward and reverse TCP connections on link L,
   and could have a wide range of values (e.g., large for an access link
   from a web server, and small for an access link to a web server).
   For this scenario, the ratio W/W1 is largely a function of the
   different levels of congestion on the forward and reverse paths.

   To explore the possibilities, we will consider some of the range of
   congestion control mechanisms for the congested link.  First, we
   consider scenarios where the limitation on the congested path is in
   the link bandwidth in bytes per second.

   Cases (1), (2), (3), (5), and (7) below represent the best scenarios
   for ACK congestion control, where the fraction of packet drops for
   TCP ACK packets roughly matches the TCP ACK packets' contribution to
   congestion.  (In several of these cases this is, at best, a rough
   match because the data packets are a factor in the bandwidth and in
   the queue limitations, while the TCP ACK packets are only a factor in
   the queue limitations.)  Cases (4) and (8) below represent
   problematic scenarios where the fraction of packet drops for TCP ACK
   packets is much higher than the TCP ACK packets' contribution to
   congestion (in terms of taking space in a congested queue, using
   scarce CPU cycles at the congested router, or using scarce
   bandwidth).  Case (6) below represents scenarios where ACK congestion
   control would not be effective because it would not be invoked.  In
   the scenarios in case (6), the fraction of packet drops for TCP ACK
   packets would be much smaller than the TCP ACK packets' contribution
   to congestion.
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   (1) The Drop-Tail queue for link L is measured in packets.  In this
       case, the congested queue can accommodate N packets (regardless
       of packet size), there is a limitation of both bandwidth in bytes
       per second and also in queue space in packets, and large data
       packets and small TCP ACK packets should see similar packet drop
       rates.  Although TCP ACK packets most likely aren't a major
       factor in the bandwidth limitation, they can be a significant
       contribution to the limitation of queue space.  So, while the
       packet drop rate for ACK packets could be high in times of
       congestion, the ACK packets are contributing to that congestion
       somewhat by using scarce buffer space.

   (2) The Drop-Tail queue is measured in bytes.  In this case, the
       congested queue can accommodate M bytes of packets, and TCP ACK
       packets don't make a significant contribution to either the
       bandwidth limitation or to the limitation in queue space.  It is
       also the case that, in this scenario, even if there is heavy
       congestion, the packet drop rate for TCP ACK packets should be
       small (because small ACK packets can often find space on the
       congested queue when large data packets can't find space).  In
       this case, ACK congestion control should not present any
       problems; the TCP ACK packets aren't contributing significantly
       to congestion and aren't experiencing significant packet drop
       rates.

   (3) The RED queue is in packet mode and is measured in packets.  This
       is similar to case (1) above.  Because the queue is measured in
       packets, small TCP ACK packets contribute to the limitation in
       queue space but not to the limitation in link bandwidth.  Because
       the queue is in packet mode, large data packets and small TCP ACK
       packets should see similar packet drop rates.

   (4) The RED queue is in packet mode but is measured in bytes.
       Because the queue is measured in bytes, small TCP ACK packets
       don't contribute significantly to either the limitation in queue
       space or to the limitation in link bandwidth.  Because the queue
       is in packet mode, large data packets and small TCP ACK packets
       should see similar packet drop rates.  If it existed, this case
       would be problematic, because the TCP ACK packets would not be
       contributing significantly to the congestion but they would see a
       similar packet drop rate as the large data packets that are
       contributing to congestion.

   (5) The RED queue is in byte mode and is measured in bytes.  This is
       similar to case (2) above.  Because the queue is measured in
       bytes, small TCP ACK packets don't contribute significantly to
       either the limitation in queue space or to the limitation in link
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       bandwidth.  At the same time, because the queue is in byte mode,
       small TCP ACK packets see much smaller packet drop rates than
       those of large data packets.

   (6) The RED queue is in byte mode but is measured in packets.
       Because the queue is measured in packets, small TCP ACK packets
       contribute to the limitation in queue space but not to the
       limitation in link bandwidth.  Because the queue is in byte mode,
       small TCP ACK packets see much smaller packet drop rates than
       those of large data packets.  If this case existed, TCP ACK
       packets would contribute somewhat to congestion but would see a
       much smaller packet drop rate than that of large data packets.

   Next, we consider scenarios where the limitation on the congested
   link is in CPU cycles at the router in packets per second, not in
   bandwidth in bytes per second.

   (7) The CPU load imposed by TCP ACK packets is similar to the load
       imposed by other packets (e.g., TCP data packets).  ACK
       congestion control would be useful in this scenario, particularly
       if TCP ACK packets saw the same packet drop rates as TCP data
       packets.

   (8) The CPU load imposed by TCP ACK packets is much less than the
       load imposed by other packets (e.g., TCP data packets).  If TCP
       ACK packets saw a smaller packet drop rate than TCP data packets,
       then the TCP ACK packet drop rate would roughly match the TCP ACK
       packets' contribution to congestion, and this would be good.  If
       TCP ACK packets saw the same packet drop rate as TCP data
       packets, this case would be problematic, because the TCP ACK
       packets would not be contributing significantly to the
       congestion, but they would see a similar packet drop rate as the
       large data packets that are contributing to congestion.

5.8. Possible Complication: TCP Implementations that Skip ACK Packets

It has been reported in IETF meetings that current TCP implementations do not always acknowledge at least every other data packet, as required by the TCP specifications. In particular, it has been reported that if a TCP receiver receives many data packets in a burst, before it is able to send an acknowledgement, then it might send a single acknowledgement for the burst of packets. We note that such a behavior would cause complications for a TCP connection that used ACK congestion control, as the sender would not be able to determine when an ACK packet had been dropped in the network or when the packet had been skipped by the receiver because it was processing a burst of data packet arrivals.
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   One possibility for addressing this problem would be for TCP
   receivers using ACK congestion control to be required to send an
   acknowledgement for each R packets, for ACK Ratio R.  In this case,
   if the receiver received a large burst of data packets back-to-back,
   the receiver would be required to send a responding burst of ACK
   packets, one for each set of R data packets.

   A second possibility for addressing this problem would be to define a
   TCP option or flag that the TCP receiver could use when sending an
   ACK packet to inform the sender that the TCP receiver `skipped' some
   ACK packets, so that the sender should not infer ACK loss if some
   previous ACK packets seem to be missing.

   Future work will explore the costs and benefits of these two
   approaches.

5.9. Possible Complication: Router or Middlebox-Based ACK Mechanisms

One possible complication would be the interaction of ACK congestion control with router-based or middlebox-based ACK mechanisms, such as ACK filtering along the reverse path ([BPK97], [WWCM99], [BA03], [KLS07]). We are not aware of the deployment of ACK filtering in the Internet, but any testing of ACK congestion control would have to look for interactions with any middlebox-based mechanisms regarding ACK packets. In particular, we would consider interactions of ACK congestion control with the possible deployment of ACK filtering on satellite links, cable modems, or the like.

5.10. Possible Complication: Data-Limited Senders

The mechanism for adjusting the ACK Ratio is designed with the goal of having the TCP receiver send at least two ACKs in response to each window of at least four full-sized data packets. However, with ACK congestion control in combination with a data-limited sender, it is possible for the sender to send at least four full-sized data packets in a round-trip time, with the receiver sending less than two ACKs in response. As an example, consider a connection where the sender's congestion window W is greater than four and the ACK Ratio R is at its maximum value of W/2. If the sender becomes data-limited and sends less than W data packets in a round-trip time, then the receiver can send less than two ACK packets in response. This behavior makes the connection more sensitive to the loss of an occasional ACK packet. Of course, there is still the safety mechanism of the receiver sending an ACK packet when the delayed ACK timer expires. However, more work would be useful to explore the conflicting goals of a
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   congestion-controlled ACK flow and a timely ACK response to the
   sender for the specific case of a connection with a data-limited
   sender and a congested ACK path.

5.11. Other Issues

Are there any problems caused by the combination of two-way traffic and reordering? Or other issues that have not yet been addressed?

6. Evaluating ACK Congestion Control

Evaluating ACK congestion control will have two components: (1) evaluating the effects of ACK congestion control on an individual TCP connection, and (2) evaluating the effects of ACK congestion control on aggregate traffic (including the effects of ACK congestion control on the aggregate congestion of the path). The first part, evaluating ACK congestion control on the performance of an individual TCP connection, will have to examine those scenarios where ACK congestion control might help the performance of a TCP connection and those scenarios where the use of ACK congestion control might cause problems. The second part, evaluating the effects of ACK congestion control on aggregate traffic, should consider scenarios where the use of ACK congestion control helps all of the connections sharing a path by reducing the aggregate congestion on the path. This part should also see if there are scenarios where ACK congestion control causes problems by increasing the burstiness of aggregate traffic or by otherwise changing traffic dynamics.

6.1. Contention in Wireless Links or in Non-Switched Ethernet

One possible benefit of ACK congestion control is that it could reduce contention in wireless links, shared Ethernet, or other environments with contention between forward-path and reverse-path traffic ([AJ03], [KIA07]). At the same time, contention on the shared medium won't necessarily result in dropped ACK packets, and therefore wouldn't necessarily be detected by ACK congestion control.

6.2. Keep-Alive and Other Special ACK Packets

Some TCP hosts send keep-alive packets when no data or ACK packets have been received over a long period of time [KEEP-ALIVE]. This keep-alive mechanism is not addressed in TCP specifications. However, such keep-alive packets, if used, should not interact with ACK congestion control one way or another. For ACK congestion control, the ACK Ratio is set small enough to allow the receiver to
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   generally send at least two ACKs for a window of data.  In addition,
   the receiver uses a delayed ACK timer with the ACK Ratio, always
   sending an acknowledgement if the delayed ACK timer expires.  Thus,
   ACK congestion control will never cause the receiver to delay
   indefinitely in sending an acknowledgement for a received data
   packet.

   Some TCP implementations send pure ACK packets as window probes, to
   solicit an ACK packet from the other end with current window
   information.  Such ACK packets will generally be orthogonal to the
   ACK congestion control specified in this document.

   TCP receivers also can send pure ACK packets as window update packets
   announcing a new value for the receive window, even when the
   acknowledgement number and SACK options in the ACK packet are not
   new.  The receiver may send window update packets even if the ACK
   congestion control mechanism would say that it is not time yet to
   send a pure ACK.  The sender will not necessarily be able to detect
   the loss of a window update ACK packet.

7. Measurements of ACK Traffic and Congestion

There are a number of studies about the traffic composition on various links in the Internet, reporting the fraction of bandwidth used by TCP data and by TCP ACK traffic [Studies]. Are there any studies that show the relative packet drop rates for TCP data and ACK traffic, for particular links or for particular TCP connections? Are there any studies of congested links that show the fraction of traffic on the congested link, or in the congested queue, that consist of TCP ACK packets?

8. Acknowledgement Congestion Control in DCCP's CCID 2

In the transport protocol DCCP [RFC4340], the congestion control mechanism for the CCID 2 profile is based on that of TCP. This section briefly discusses some of the issues that have been addressed in the acknowledgement congestion control already standardized in CCID 2 [RFC4341].
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   Rate-based pacing:

      For CCID 2, RFC 4341 says that "senders MAY use a form of rate-
      based pacing when sending multiple data packets liberated by a
      single ACK packet, rather than sending all liberated data packets
      in a single burst."  However, rate-based pacing is not required in
      CCID 2.

   Increasing the congestion window:

      For CCID 2, RFC 4341 says that "when cwnd < ssthresh, meaning that
      the sender is in slow-start, the congestion window is increased by
      one packet for every two newly acknowledged data packets with ACK
      Vector State 0 (not ECN-marked), up to a maximum of ACK Ratio/2
      packets per acknowledgement.  This is a modified form of
      Appropriate Byte Counting [RFC3465] that is consistent with TCP's
      current standard (which does not include byte counting), but
      allows CCID 2 to increase as aggressively as TCP when CCID 2's ACK
      Ratio is greater than the default value of two.  When cwnd >=
      ssthresh, the congestion window is increased by one packet for
      every window of data acknowledged without lost or marked packets."

9. Security Considerations

What are the sender's incentives to cheat on ACK congestion control? What are the receiver's incentives to cheat? What are the avenues open for cheating? As long as ACK congestion control is optional, neither host can be forced to use ACK congestion control if it doesn't want to. So ACK congestion control will only be used if the sender or receiver have some chance of receiving some benefit. As long as ACK congestion control is optional for TCP, there is little incentive for the TCP end nodes to cheat on non-ECN-based ACK congestion control. There is nothing now that requires TCP hosts to use congestion control in response to dropped ACK packets. What avenues for cheating are opened by the use of ECN-Capable ACK packets? If the end nodes can use ECN to have ACK packets marked rather than dropped, and if the end nodes can then avoid the use of ACK congestion control that goes along with the use of ECN on ACK packets, then the end nodes could have an incentive to cheat. Senders could cheat by not instructing the receiver to use a higher ACK Ratio; the receiver would have a hard time detecting this cheating. Receivers could cheat by not using the ACK Ratio they were instructed to use, but senders could easily detect this cheating. However, receivers could also cheat by not using ACK congestion
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   control and still sending ACK packets as ECN-Capable, so ACK
   congestion control is not a necessary component for receivers to
   cheat about sending ECN-Capable ACK packets.  One question would be
   whether there is any way for receivers to cheat about sending ECN-
   Capable ACK packets and not using appropriate ACK congestion control
   without this cheating being easily detected by the sender.

   What about the ability of routers or middleboxes to detect TCP
   receivers that cheat by inappropriately sending ACK packets as ECN-
   Capable?  The router will only know if the receiver is authorized to
   send ACK packets as ECN-Capable if the router can see traffic on both
   the forward and reverse paths and monitored both the SYN and SYN/ACK
   packets (and was able to read the TCP options in the packet headers).
   If ACK congestion control has been negotiated, the router will only
   know if ACK congestion control is being used correctly by the
   receiver if it can monitor the ACK Ratio options sent from the sender
   to the receiver.  If ACK congestion control is being used, the router
   will not necessarily be able to tell if ACK congestion control is
   being used correctly by the sender, because drops of ACK packets
   might be occurring after the ACK packets have left the router.
   However, if the router sees the ACK Ratio options sent from the
   sender, the router will be able to tell if the sender is correctly
   accounting for those ACK packets that are dropped or ECN-marked on
   the path from the receiver to the router.

10. IANA Considerations

No IANA action is needed at this time. If this document was advanced as Experimental or Proposed Standard, then IANA would allocate the option numbers for the two TCP options, the ACK Congestion Control Permitted option, and the ACK Ratio option. In such a case, the following two lines would be added to the TCP Option Numbers registry (maintained by IANA -- http://www.iana.org): Kind Length Meaning Reference ---- ------ --------------------------------- ----------- TBD1 2 ACK Congestion Control Permitted [RFCXXXX] TBD2 3 ACK Ratio [RFCXXXX] In the absence of TCP option numbers allocated by IANA, experimenters may use the TCP Option Numbers set aside for Experimentation in RFC 4727 [RFC4727]. As stressed in Section 1 of RFC 3692 [RFC3692], the TCP Option Numbers in the experimental range are intended for experimentation and testing and not for wide or general deployments; these option numbers could be in use by other experimentors for other purposes.
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11. Conclusions

This document specifies a congestion control mechanism for acknowledgement (ACKs) traffic for TCP and discusses the possible complications. We are deferring a recommendation on the use of this mechanism for TCP until it has been evaluated more fully.

12. Acknowledgements

Many thanks for feedback from Mark Allman, Armando Caro, Alfred Hoenes, Ilpoo Jarvinen, Sara Landstrom, Anantha Ramaiah, and Michael Welzl, and for contributed text from Michael Welzl.
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Appendix A. Related Work

There exist several papers dealing with controlling congestion in the reverse path of a TCP connection, especially in the context of networks with bandwidth asymmetry. Some of these proposals require explicit support from routers or middleboxes, whereas others are "pure" end-to-end schemes. RFC 3449 [RFC3449] discusses TCP performance problems that arise in TCP connections over asymmetric paths. Section 3 of RFC 3449 describes in detail how congestion on the ACK path can affect overall TCP performance. RFC 3449 also outlines a number of proposed mitigations, including ACK congestion control. The experimental ACK congestion control mechanism discussed in that RFC relies on ECN, with the TCP sender detecting congestion on the ACK path from ECN- marked packets. RFC 3449 also discusses two receiver-based mechanisms, the Window Prediction Mechanism (WPM) [CLP98] and Acknowledgement based on Cwnd Estimation (ACE) [MJW00], for using a dynamic ACK Ratio. RFC 3449 also considers link- and network-layer techniques that address congestion on the upstream path. These include header compression as well as bandwidth management techniques for the upstream link, including ACK filtering and ACK reconstruction. RFC 3135 [RFC3135], "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", surveys a range of Performance Enhancing Proxies used to improve TCP behavior, including proxies for ACK filtering and reconstruction. RFC 2760 [RFC2760], "Ongoing TCP Research Related to Satellites", discusses both ACK congestion control and ACK filtering and reconstruction, with detailed descriptions of the mechanisms proposed by Balakrishnan, et al. in [BPK97]. Landstrom, et al. in [LL07] explore a mechanism where the receiver sends only four acknowledgements per window of data, along with the sender using a form of Appropriate Byte Counting. In addition, the receiver reverts to a lower acknowledgement frequency after a timeout, to avoid unnecessary retransmit timeouts. One conclusion of the paper is that pacing at the sender introduces an additional delay and might not be necessary. A key result of the paper is that, with the use of some form of byte counting at the sender, it is possible for TCP to use a lower acknowledgement frequency than that of delayed acknowledgements.
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A.1. ECN-Only Mechanisms

Balakrishnan, et al. in [BPK97] describe the use of ECN to detect congestion in the return path, in order to reduce the sending rate of ACKs. The use of a RED queue in the reverse path allows for marking of ACK packets. The sender echoes back ECN congestion marks to the receiver. The receiver keeps an ACK Ratio d (called the "delayed-ACK factor"), specifying the number of data segments that have to be received before the receiver sends a new ACK. The ACK Ratio d is managed using multiplicative-increase, additive-decrease; upon reception of a congestion mark, the receiver doubles the value of d (hence dividing the ACK sending rate by two). The ACK Ratio decreases linearly for each RTT in which no ECN-marked ACKs are received. Multiple congestion marks received in an RTT are treated as a single congestion event, i.e., d can be doubled at most once per RTT. The TCP timestamp option is used to keep track of the RTT values.

A.2. Receiver-Only Mechanisms

In [MJW00], Tam Ming-Chit, et al. propose a receiver-based method for calculating an "appropriate" number of ACKs per congestion window (cwnd) of data, in order to alleviate congestion on the reverse path. The sender's cwnd is estimated at the receiver by counting the number of received packets per RTT (which also has to be estimated by the receiver). From this estimate, a simple algorithm is used to compute the number of ACKs to be sent per cwnd. The algorithm enforces a lower bound on the number of ACKs per cwnd, aiming at minimizing the probability of timeout at the sender due to ACK loss. Similarly, the ACK Ratio is upper-bounded so as to avoid excessive ACK delay. Blandford, et al. [BGG+07] propose an end-to-end, receiver-oriented scheme called "smartacking". The algorithm is based upon the receiver's monitoring the inter-segment arrival time for data packets and adapting the ACK sending rate in response. When the bottleneck link is underutilized, ACKs are sent frequently (up to one ACK per received segment) to promote fast growth of the congestion window. On the other hand, when the bottleneck is close to full utilization, the algorithm tries to reduce control traffic overhead and slow congestion window growth by generating ACKs at the minimum rate needed to keep the data pipe full. Reducing the number of ACKs (or, equivalently, increasing the amount of bytes acknowledged by each ACK) can increase the burstiness of the TCP sender. Hence, any mechanism as those cited above should be coupled with a burst mitigation technique, such as rate-based pacing, that paces the sending of data segments ([AB05], [ASA00], [BPK97]).
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A.3. Middlebox-Based Mechanisms

ACK filtering (AF) [BPK97] from Balakrishnan, et al. is a router- based technique that tries to reduce the number of ACKs sent over the congested return link. With AF, an arriving ACK may replace preceding, older ACKs at the bottleneck queue. An aggressive replacement policy might guarantee that at most one ACK per connection is waiting in the queue, alleviating congestion. However, as in other proposals, care must be taken to avoid sender timeouts in case the (too few) ACKs resulting from the filtering get lost. The idea of filtering ACKs has been extended in [YMH03] to deal with SACK information. Aweya, et al. [AOM02] present a middlebox-based approach for mitigating data packet bursts and for controlling the uplink ACK congestion. The main idea is to perform pacing on ACK segments on an edge device close to the sender, so as to control the ACK arrival rate at the sender.

Appendix B. Design Considerations

B.1. The TCP ACK Ratio Option or an AckNow Bit in Data Packets?

In the ACK congestion control mechanism specified in this document, the sender uses the TCP ACK Ratio option to tell the receiver the ACK Ratio to use. An alternate approach to the TCP ACK Ratio option could be for the sender to use an AckNow bit in the TCP header of data packets, telling the receiver to acknowledge this data packet. In the discussion below, we call these two approaches the TCP ACK Ratio option approach and the AckNow approach. An advantage of an AckNow approach is that it would require less state from the receiver; the receiver would not need to maintain a variable for the current ACK Ratio and would not need to keep track of the number of data packets un-ACKed to date. However, a disadvantage of the AckNow approach is that the sender does not know when packets will be reordered, delayed, or dropped on the path to the receiver. In particular, the sender does not have control over whether a data packet with the AckNow bit set is reordered, delayed, or dropped in the network. For this reason, we have chosen the approach of the sender determining the ACK Ratio that should be used and sending the ACK Ratio to the receiver, rather than the sender telling the receiver exactly which data packets to acknowledge.
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   An additional disadvantage of the AckNow approach is that it would
   add complications and difficulties for the default cases of the
   receiver using an ACK Ratio of one or two, as is done in the absence
   of ACK congestion control.

   For these reasons, we have specified that the sender determines the
   ACK Ratio to use and tells the receiver, rather than the sender
   setting an AckNow bit in the TCP Header of selected data packets.

Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte Counting (ABC)", RFC 3465, February 2003. [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, January 2004. [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, March 2006. [RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion Control Protocol (DCCP) Congestion Control ID 2: TCP-like Congestion Control", RFC 4341, March 2006. [RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006. [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, September 2009.

Informative References

[RFC2760] Allman, M., Ed., Dawkins, S., Glover, D., Griner, J., Tran, D., Henderson, T., Heidemann, J., Touch, J., Kruse, H., Ostermann, S., Scott, K., and J. Semke, "Ongoing TCP Research Related to Satellites", RFC 2760, February 2000. [RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z. Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations", RFC 3135, June 2001.
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   [RFC3168]    Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
                of Explicit Congestion Notification (ECN) to IP", RFC
                3168, September 2001.

   [RFC3449]    Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
                Sooriyabandara, "TCP Performance Implications of Network
                Path Asymmetry", BCP 69, RFC 3449, December 2002.

   [RFC4653]    Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
                "Improving the Robustness of TCP to Non-Congestion
                Events", RFC 4653, August 2006.

   [ASA00]      Aggarwal, A., Savage, S., and T. Anderson,
                "Understanding the Performance of TCP Pacing", INFOCOM
                (3), pp. 1157-1165, 2000.

   [AB05]       Allman, M., and E. Blanton, "Notes on Burst Mitigation
                for Transport Protocols", SIGCOMM, Computer
                Communications Review, 35(2):5360, 2005.

   [AJ03]       Altman, E., and T. Jimenez, "Novel Delayed ACK
                Techniques for Improving TCP Performance in Multihop
                Wireless Networks", Proc. of the Personal Wireless
                Communications, 2003.

   [AOM02]      Aweya, J., Ouellette, M., and D. Y. Montuno, "A Self-
                regulating TCP Acknowledgement (ack) Pacing Scheme",
                International Journal of Network Management,
                12(3):145163, 2002.

   [BA03]       Barakat, C., and E. Altman, "On ACK Filtering on a Slow
                Reverse Channel", International Journal of Satellite
                Communications and Networking, V.21 N.3, 2003.

   [BPK97]      Balakrishnan, H., Padmanabhan, V., and Katz, R., "The
                Effects of Asymmetry on TCP Performance", Third ACM/IEEE
                Mobicom Conference, September 1997.

   [BGG+07]     Blandford, D.K., Goldman, S.A., Gorinsky, S., Zhou, Y.,
                and D.R. Dooly, "Smartacking: Improving TCP Performance
                from the Receiving End", Journal of Internet
                Engineering, 1(1), 2007.

   [CLP98]      Calveras, A., Linares, J., and J. Paradells, "Window
                Prediction Mechanism for Improving TCP in Wireless
                Asymmetric Links". Proc. IEEE Global Communications
                Conference (GLOBECOM), Sydney Australia, pp. 533-538,
                November 1998.
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   [KIA07]      Keceli, F., Inan, I., and E. Ayanoglu, "TCP ACK
                Congestion Control and Filtering for Fairness Provision
                in the Uplink of IEEE 802.11 Infrastructure Basic
                Service Set", Proc. IEEE ICC 2007, June 2007.

   [KEEP-ALIVE] Busatto, F., "TCP Keepalive HOWTO", May 2007,
                http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/index.html.

   [KLS07]      Kim, H., Lee, H., and S. Shin, "On the Cross-Layer
                Impact of TCP ACK Thinning on IEEE 802.11 Wireless MAC
                Dynamics", IEICE Transactions on Communications, 2007.

   [LL07]       Landstrom, S., and Larzon, L.A., "Reducing the TCP
                Acknowledgement Frequency", SIGCOMM, Computer
                Communications Review, July 2007.

   [MJW00]      Ming-Chit, I.T., Jinsong, D., and W. Wang, "Improving
                TCP Performance Over Asymmetric Networks", SIGCOMM,
                Computer Communications Review (CCR), Vol.30, No.3,
                2000.

   [Studies]    Floyd, S., "Measurement Studies of End-to-End Congestion
                Control in the Internet",
                http://www.icir.org/floyd/ccmeasure.html.

   [WWCM99]     Wu, H., Wu, J., Cheng, S., and J. Ma, "ACK Filtering on
                Bandwidth Asymmetry Networks", IEEE Communications,
                1999.

   [YMH03]      Yu, L., Minhua, Y., and Z. Huimin, "The Improvement of
                TCP Performance in Bandwidth Asymmetric Network", IEEE
                PIMRC, 1:482-486, September 2003.
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Authors' Addresses

Sally Floyd ICSI Center for Internet Research 1947 Center Street, Suite 600 Berkeley, CA 94704 USA EMail: floyd@icir.org Andres Arcia Networking, Security & Multimedia (RSM) Universidad de Los Andes TELECOM Bretagne Facultad de Ingenieria Rue de la Chataigneraie, CS 17607 Nucleo La Hechicera 35576 Cesson Sevigne Cedex Merida, Merida 5101 France Venezuela EMail: ae.arcia@telecom-bretagne.eu EMail: amoret@ula.ve URI: http://www.ula.ve David Ros Networking, Security & Multimedia (RSM) Dpt. TELECOM Bretagne Rue de la Chataigneraie, CS 17607 35576 Cesson Sevigne Cedex France EMail: David.Ros@telecom-bretagne.eu Janardhan R. Iyengar Math and Computer Science Franklin & Marshall College P. O. Box 3003 Lancaster, PA 17604-3003 USA EMail: jiyengar@fandm.edu